Vena cava filters are used to capture potentially fatal pulmonary emboli at an anatomical location where they may pose less risk of pulmonary embolism for a patient. Since the vast majority of pulmonary emboli originate from the lower body, filters are mainly placed in the inferior vena cava (IVC).
Vena cava filters have been in use since the 1960s in a variety of configurations. Early filters required open surgical placement (Mobin—Uddin Filter; Kimray-Greenfield filter). Since the late 1970s, improvements in delivery were made and numerous filters were developed for minimally invasive percutaneous placement. These filters included the Greenfield filter, the Gianturco Bird's Nest Filter, the Vena Tech LGM filter, the Simon Nitinol filter and others.
Although addressing some desirable characteristics of a filter, the majority of the IVC filters presently on the market do not satisfy other desirable characteristics of an ideal filter. One of the attributes of an ideal vena cava filter is ease of deployment using minimally invasive percutaneous techniques as well as the ability of the device to provide optimal filtration while remaining centered within the vessel. Unfortunately, many filters are designed for ease of deployment but they either do not remain aligned within the vessel or have sub-optimal filtering capabilities.
An ideal device should capture blood clots while ensuring continued blood flow through the vessel. Blood flow disruption and turbulence often leads to thrombus formation and buildup at and around the filter. Studies have demonstrated that a conical filter configuration provides the optimal filtering efficiency. Filtering efficiency, for the purposes of this invention, can be defined as the capability of the device to capture and retain clots of a pre-determined size, the ability to maintain blood flow through the filter in the presence of captured clots, and the capability of dissolving or lysing the clots caught in the filter. Conical designs force clots toward the center of the filter, allowing blood flow passage around the clot. Continued flow of blood through the filter when a clot load is present ensures that captured clots are exposed to the lysing action of the blood flow.
Although conical filter configurations currently available on the market provide optimal filtering capabilities, these designs are prone to tilting and misalignment. When not in proper alignment, filtering ability is compromised. Misalignment can also lead to filter leg crossing, vessel perforation and migration of the device due to incomplete vessel wall engagement. Laminar blood flow is disturbed, effective lysing of captured clots decreases, and thrombus build-up occurs.
To address the misalignment problem, filtering cones have been designed with alignment mechanisms to prevent tilting. It is possible to build a simple centering cage base/conical filter combination design by attaching the base to the filter segment in series. This design, while exhibiting increased stability, is not practical due to the increased length of the device. The desired length of a typical IVC filter is between 3 and 5 centimeters. Longer lengths are undesirable because of the limited implantation space of the vena cava. For example, in some cases it is necessary to deploy a second filter due to malfunction of the initially placed filter. Shortening the filter segment may make the overall device length acceptable, but may result in sub-optimal filter strut angles. Alternatively, shortening the centering cage segment may compromise the alignment function of the device.
As with all permanent implant devices, the optimal device design maintains structural integrity of the device for the duration of implantation. Although rare, filters can develop fractures which have potentially fatal complications including filter migration into the right atrium and pulmonary embolism caused by compromised filtering efficiency. In addition to long term performance characteristics, it is desirable to provide an IVC filter that is simple and inexpensive to manufacture without requiring complicated assembly processes that might compromise the long-term integrity of the device or increase the overall cost of the device. IVC filter devices should also be sufficiently low profile to be delivered through a small diameter delivery system to minimize insertion site complications.